Her grueling schedule has paid off; she has only lost twice to U.S. opponents, once when she was 12 and once to Christina Cruz, her toughest foe, in 2011. Each time, she retooled. "From the first loss to Johanna Mendez, I learned to train harder," she says. "From the second loss to Christina Cruz, I learned to fight smarter."

During training, which is six days a week (down from seven after a trainer convinced Silva the body needs a day of rest to get stronger), Esparza gets up early to swim and work on strength training. Lunch is pork loin or chicken breast and vegetables (she put on six pounds of muscle to move up to the 112-pound class, the lowest for women boxers). Then, she often naps. Silva meets her at the gym at the Fellowship of Nations in Houston, where she began training last year after the gym they started at more than a decade ago closed. She jumps rope, hits the bags and spars under his watchful eye. Later, she goes for a run of anywhere from a mile to four miles.

Esparza punched her ticket to London by winning the U.S. Olympic trials, going undefeated and beating Washington D.C.'s Tyriesha Douglas, 32-17, in the final. Then she had to finish as the highest flyweight at the world championships in China in May where eight boxers from the weight class earned automatic Olympic berths. She started fast, defeating Argentina's Paola Benavidez, 20-10, in the first round before downing Vietnam's teenage phenom, Luu Thi Duyen, 28-13. But her win streak stopped when Esparza dropped a 16-8 decision to Ren Cancan of China, the eventual world champion now ranked number one among flyweights.

Like other losses, Esparza says she learned from this one and will come back stronger. She's been boxing internationally since she was 16, so she knows her opponents well. She’s taken to listening to hymns lately as part of her pre-fight routine.

She wants to be calm, dissecting her opponents, always thinking, always seeking the peace of mind that will let her do her best. "In order to seek calmness, you have to try and outsmart your opponent and not just throw punches aimlessly," she adds.

"In London, I will have to beat [Cancan] by fighting a smarter fight and have a more technical approach," she says. "When I fought her in China, I had already qualified for London so mentally I was not the same as I was in the other rounds. This time I will be more mentally prepared."